It has previously been proposed to provide an ignition control system in which a signal generator generates an undulating signal which is applied to a threshold switch. When the threshold switch senses a first flank of the signal, for example a rising flank, it provides a closing signal to an electronic switch to close the circuit through an ignition coil. Current will thus flow through the ignition coil storing electromagnetic energy therein. When the threshold switch senses a threshold level, which may be different than the first, from the other flank of the signal, that is, from the trailing flank, an ignition trigger is generated to open the switch through the ignition coil, thus inducing a sharp voltage pulse, causing an ignition event in the secondary of the ignition coil.
Various devices and circuits may be used to change the relative occurrence of the ignition instant with respect to the crankshaft position of the engine. In the system described, this involves a change in the signal or the threshold level of the threshold switch, for example to advance the spark with respect to top dead center (TDC) position of a piston of the internal combustion engine. In the referenced publication, German Disclosure Document DE-OS 25 49 586, there is described such an ignition system in which an arrangement is provided to turn off the threshold switch, thus causing an ignition event to occur in the lower range of the speed range of the internal combustion engine. Thus, an ignition event will be triggered at approximately the same angular position--with respect to TDC of the piston--unless some circuits or devices cause an advance of the triggering of the ignition event. Regardless of the presence of such advancing signals or devices, however, an ignition event should always be triggered at the latest at this predetermined time with respect to TDC position of the piston. This predetermined time may, for example, by just in advance of or at the TDC position.